Photolithography is a common term which describes a process of surface mask creation above a semiconductor substrate for patterning of underlying layers such as the bulk semiconductor substrate and different dielectric, metal or polycrystalline semiconductor films or for implanting of special impurities into certain regions of the semiconductor substrate.
Basically, organic films containing photo-sensitive compounds are used as a mask pattern. Under light exposure, these compounds change their characteristics and allow removal of exposed portions of the organic photoresist by special solvents (as in case of so-called positive type photoresist) which do not attack unexposed or masked portions of the photoresist.
Thus, the residual organic photoresist materials are removed before etching of the underlying layers is performed. There was a number of problems at this time caused by general physical features of the organic materials. One of the problems was caused by the requirement of heating the silicon substrate during developing of the photoresist, which resulted in the bottom portions of the organic materials being baked differently from the top layer. At the same time, due to overexposure limitations of the organic photoresists, residual organic resist material, or photoresist footing, was left at the base of the photoresist mask after developing. The photoresist footing resulted critical dimensions (CDs) being smaller than desired according to the photoresist mask. Thus, feature size, photoresist type, baking conditions, underlying film features, etc. had to be taken into account in developing the photoresist.
At this time, plasma processing came into use to remove the organic photoresist. First, only pure oxygen plasma was used and it was created in bulk quartz reactors. Later, when dry etch of semiconductor materials appeared, the pure oxygen plasma began to be used in situ in parallel plate reactors.
However, the CD of integrated circuits are becoming smaller and smaller. To provide necessary resolution in order to meet the requirements of sub-half-micron design rules, deep ultraviolet (DUV) light with wave lengths less than 300 nm came into use for the smallest CD integrated circuits. This in turn required photoresists opaque to DUV illumination. However, most photoresists are transparent to DUV illumination. This transparency caused overexposure of bottom layers of DUV photoresists by light reflected from the substrate surface and led to loss of line width control due to more extensive removal of the overexposed photoresist.
To suppress overexposure, special anti-reflection coatings (ARC) came into use to be applied under photoresists to allow more accurate patterning. These ARC films reduced reflection drastically and made sub-half-micron size features reproducible.
There are several types of ARC that were used in integrated circuit fabrication. They include: organic ARCs, titanium nitride films, and multi-layer silicon oxi-nitride films.
The most flexible of the ARCs are organic ARCs that can be applied to the masking of microelectronic materials such as silicon, dielectrics, and metals. Most materials used in microelectronics cause organic photoresist poisoning, namely depleting or enriching proton concentration in the pre-surface layers of DUV photoresists. Since organic ARCs are chemically similar to photoresists, they do not cause poisoning of DUV photoresists and have come into common use.
One more advantage of organic ARCs is that they can be easily incorporated into the photolithographic process. In general, organic ARCs are applied to the wafer surface by a spin-deposition process before photoresist coating. Because of the simplicity of spin-deposition, the equipment for spinning-on organic ARC can be easily installed in the photolithographic equipment.
However, the organic ARC must be removed using the photoresist mask before the underlying silicon, dielectrics, or metals could be processed or etched and this is a major problem because organic ARCs are resistant to photoresist developers and remain untouched after photoresist developing.
Further, due to the thickness of the organic ARCs, there have been various problems associated with their removal without substantially changing the profiles of the openings in the photoresist masks.
Originally, plasma discharge in pure oxygen or oxygen diluted with a noble gas were used in "descum" processes to remove photoresist traces prior to wet oxide etch. These descum processes could be performed either ex-situ, outside the etching equipment or in-situ, within the equipment. Ex-situ descum processes have two main disadvantages from a manufacturing point of view even where the processes are otherwise excellent. First, the additional operations required reduce throughput of manufacturing line and second, yield is reduced because of defects added during loading and unloading the wafers. Therefore, in-situ descum processes are preferred.
In both descum processes, it was found that pure oxygen plasma is very agressive against organic materials and has great efficiency with respect to interaction with organic material, but since the etching process is based on producing active atomic oxygen, it is impossible to obtain a true anisotropic, unidirectional, etch. The atomic oxygen attacked the side walls of photoresist pattern and led to loss in line width control and uniformity. The processes were very sensitive to pattern density as well.
When organic ARCs came to be used, the above plasma processes were first used in the removal of the organic ARCs and the same problems occurred as when using the processes for descum.
The next steps in the development of organic ARC removal processes applied fluorocarbon compounds, classified as freons. Simple freons were used such as hexafluorethane (Freon-216), or tetrafluoromethane (Freon-14), or trifluoromethane (Freon-23). These freons gases were also used with oxygen. Contrary to pure oxygen plasma, these types of plasma discharge give better uniformity and, as a result, better line width control. But these methods have some disadvantages because, under radio frequency (RF) glow discharge conditions, molecules of Freon-14 and oxygen dissociate as below: EQU CF.sub.4 +e.fwdarw.CF.sub.3.sup.(+) *+F*+(2)e (1) EQU O.sub.2 +e.fwdarw.O.sup.(+) *+O*+(2)e (2)
where the asterisks represent excited neutral atoms and radicals.
It is known that excited atomic fluorine and atomic excited oxygen are very agressive etchants against organic materials, and when they are produced in a volume of plasma and delivered to the substrate surface, they provide high velocity isotropic chemical reaction even without any additional energy applied to the surface.
F*+organic.fwdarw.HF(.uparw.), CF*(.uparw.), CHF*(.uparw.), CH.sub.2 F*(.uparw.), CH.sub.3 F*(.uparw.), CF.sub.2 *(.uparw.) (3) EQU O*+organic.fwdarw.OH(.uparw.), H.sub.2 O(.uparw.), CO(.uparw.), CO.sub.2 (.uparw.) (4)
These reactions provide mask profiles with the problematic ARC footing, or residual ARC material, at the bottom of the developed windows of ARCs. This created the same problems as mask footing. Furthermore, when the ARC was cleared from the underlying film, it was exposed to attack of the fluorine containing plasma. This was a major problem since silicon based materials are widely used in semiconductor manufacturing which are undesirably etched by fluorine containing plasmas during the extra-etching applied to remove the ARC footing.
To overcome loss in line width control and to prevent polymerization on pattern side walls, processes without oxygen utilizing helium plasma, nitrogen plasma, or glow discharge in their mixture were developed. These processes were free from polymerization and, contrary to fluorocarbon compound based plasmas, they did not produce any shelter effect and even provided a kind of taper profile of the photoresist pattern. They could keep CD control but a major problem was lack of productivity.
Etch rates obtained in these processes were an order of magnitude lower than processes based on the chemical reaction between oxygen or fluorine with organic materials because these processes relied only ion bombardment for their effect. Distribution of ions by energy in glow discharge was such that most of the ions have insufficient energy to provide sputtering. Thus, only very small number of ions contribute to the efficiency of the process.
Other organic ARC removal processes used glow discharge in single gas, such as carbon monoxide, carbon dioxide, lower nitrogen oxide, nitrogen monoxide, or discharge in the mixture of these gases with nitrogen or noble gas such as helium, argon and neon.
It has been observed that carbon dioxide gas, and especially carbon monoxide based gas mixtures, in commonly used dipole ring magnet (DRM) reactive ion etching (RIE) systems lead to polymerization instead of etching where there was a buildup of material on the photoresist and the organic ARC materials, which result in subsequent loss of CD control.
In addition, with all these processes, because of the selectivity of the process on organic materials, ARC footing occurs and detrimentally shrinks the dimensions of the openings and leads to expansion of the dark field features of integrated circuits.
Solutions to these problems have been long sought, but have long eluded those having skill in the art.